Here's the deal: most lenses out there (for Canon, Nikon, or other systems) are designed to work with 35 mm film. In other words, they project an image onto the film that is equal to the size of that particular film. The trouble is that most digital sensors--except those used in high-end digital SLRs with what are called "full frame" sensors--are smaller than 35 mm, meaning that the sensor cannot record the entire 35 mm-sized image and effectively "crops" (or cuts off) the outside edges.
This has two implications for the photographer: first, it creates some disconnect between what you see through the viewfinder (a full 35 mm frame) and the final image; and, second, it has the same effect as using a lens with greater focal length. Recall that, during our discussion of focal length, we talked about how higher focal lengths (200 mm, 300mm, 400mm, etc.) had the effect of magnifying the image and, in the process, narrowing the field of view. While crop factor doesn't magnify the image (remember: the lens still projects the same image; only the sensor has changed), it does narrow the field of view. To understand what this means in terms of the field of view, photographers use a "crop factor" or, alternatively, "focal length multiplier."
Arrrggh ... I can hear the groans already, but bear with me. An illustration should help clarify things a bit. The standard crop factor (or focal length multiplier) for the Canon system is 1.6, which means that my effective focal length is 1.6 times whatever is written on the barrel of the lens. So, for example, when I take a 20-35mm wide angle film lens and put it on my digital SLR, I have to multiply those focal lengths by 1.6 to understand what it does to my field of view. So, 20 x 1.6 = 32 and 35 x 1.6 = 56, and, just like that, I have an effective 32-56mm lens, which means that the lens is no longer really a "wide angle" lens when used on a digital camera. Still a perfectly good lens, mind you; it just doesn't offer a true wide angle perspective.
To address this problem, Canon has designed a series of EF-S lenses that work only with their digital SLR cameras and do offer a true wide angle perspective. Please note, however, that you still have to apply a crop factor to these lenses to get the 35 mm equivalent. So, for example, the 10-22mm EF-S lens is roughly the equivalent of a 16-32 mm film lens, meaning that the wide angle (anything greater than 35 mm) is restored. Another advantage of EF-S lenses is that they can use less glass than their film equivalents, so they are in general smaller and lighter. Still, Canon's EF lenses remain their standard series, and probably always will, so I personally don't plan to invest any money in EF-S lenses.
In any event, having a crop factor isn't always a bad thing. While it constrains the wide angle end of thing, it actually helps on the telephoto side, particularly with fixed focal length (prime) lenses, so my 100mm lens acts more like a 160 mm lens.
Pefectly clear? I thought so. If it isn't, take comfort in this: if you put your camera on "live view," what you see is what you get. In other words, while it's helpful to understand focal length and conversion factors and what it means for your final image, you can learn composition and take great photographs even without understanding this stuff. Trial and error will get you there as well, just a little more sloooooowly.